After having an important role in one Super Mario game (Super Mario Galaxy), Rosalina has remained with the Super Mario franchise in a number of side-rolls, including a small appearance in Super Mario Galaxy 2, being an unlockable character in Mario Kart Wii and a secret bonus character in Super Mario 3D World.
It has just been confirmed that Rosalina alongside her companion Luma will be playable characters in the upcoming Wii U and Nintendo 3DS releases of Super Smash Bros, joining other new characters such as Villager, Mega Man and Wii Fit Trainer.
You can check out some in-game footage of her in the below trailer, once you get past the minute and a half of pre-rendered promotional guff. The trailer also hints that Rainbow Road from the Mario Kart series will make an appearance as a battle stage.